An ideal or perfect Faraday cage is an active blocker of electromagnetic waves at or below the highest frequency of interest. In an ideal Faraday cage, an electromagnetic wave having a frequency within the frequency range of interests attempting to pass through the material forming the Faraday cage gives rise to a precisely countervailing electromagnetic signal, in effect canceling it out. An ideal Faraday cage is a sphere around the enclosed volume. Most enclosures which shield, deliberately or otherwise, enclosed volumes from external transmissions, or prevent transmission from the enclosed volume are in fact approximations of a Faraday cage. A volume enclosed by solid metal (e.g., a gas or liquid tank, a rocket motor housing, etc.) or by a mesh that creates an approximation of a Faraday cage (e.g., electrically shielded portions of a device, building, etc.), can be referred to as an effective Faraday cage volume. These enclosures have effects on the effective Faraday cage volume which are very similar, though not identical to, those which would be seen with an ideal Faraday cage.
Current approaches to monitor conditions and/or communicate from within such a volume require a physical breach of the enclosure structure (e.g., the solid metal or mesh), or simply storage of the data until the enclosure is opened from the exterior so that the data can be retrieved. Common approaches for physically breaching the enclosure include drilling a hole through a container, providing a radio-transparent “window” in the container (e.g., a piece of glass), and/or the like. For an enclosed effective Faraday cage volume formed by a mesh Faraday cage, a wire may be run through a space between meshes.
For many applications, prior art approaches have significant drawbacks and/or cannot be used. For example, pressurized tanks or components, such as rocket motor housings, will severely strain any defect—deliberate or otherwise—in the structure during use. Such strain can possibly break the structure, which can have potentially disastrous results. As a result, these structures must have as few flaws as possible. Installing sufficiently resilient radio-transparent windows in such structures, even if possible, is extremely expensive and painstaking work, and will still leave the original components less robust than they would have been otherwise. In some cases, the enclosure itself may also be within another enclosure, which partially or completely blocks radio frequency (RF) transmissions, further complicating the process.
Some prior art wireless communications approaches have proposed to use an electronic enclosure as an antenna. In one approach, a consumer electronics metal housing is used as an antenna to address the lack of space available for a conventional antenna. In other approaches, an antenna is embedded into a casing.